In another letter to one Major R. Bowen, Tolkien says that the Ainur take physical form similar to that of the Children of God (which he goes on to say this is an actual physical form, and not just an illusion), that taking material form requires energy of the spirit (or will power), which in Saurons case, took longer after the battle with Galgalad and Elendil than after the Downfall of Numenor. In a letter to one Rhona Beare, Tolkien says that Sauron corrupting Numenor required him expending energy, and as thus needed time for his own powers to return (this means that Sauron corrupting Numenor was not merely him spreading lies, but actually spreading his dark power, which he needed time to regain his power from). As a response, Eru reshaped the world as per the request of Manwe. There is also the implication that not even Eru can destroy spirits with free wills.